The invention relates to a device for non-destructive testing of length-adjustable, long ferromagnetic bodies for structural faults by means of magnetizing the respective body, onto which at least one magnetic field is directed and in which structural faults cause in the magnetic field pattern changes that are picked up by magnetic field sensors arranged on or near to the body surface.
Currently available testing systems working on the leakage-flux principle are designed either for longitudinal fault or for transverse fault detection during pipe testing. Rotation testing systems are used mostly for longitudinal fault testing, as they are the obvious choice on account of the required magnetization transverse to the longitudinal axis of the pipe. The magnetic field sensors intersect the stray field lines directly and therefore give an optimum signal if there are faults in the material. Transverse faults cannot be detected with a testing system of this type since these faults run in the field direction and generate practically no leakage flux which might be evaluated. For this reason a transverse fault testing system is required in addition where transverse faults too can occur in test specimens. For this test, a magnetic field is generated in the pipe axis while a stationary set of magnetic field sensors is arranged around the pipe. This means that two complete testing systems with magnetic field sensors and the means for magnetization of the test specimens are necessary for complete testing.
It is also possible to determine the leakage fluxes caused by structural faults without mechanical rotation of the test specimens. To do so, gradient sensors are arranged in at least one row along the circumference of the test specimen, transverse to its direction of transportation. The gradient sensors are connected to an electronic evaluation unit which scans the gradient sensors one after the other. To make the measurement accuracy independent of local static magnetic fields, position-dependent compensation values are assigned to the gradient sensors.
The mechanical rotation of a test sensor of a longitudinal fault testing system is replaced by electronic scanning of the individual sensors. Scanning is inertia-free and therefore does not affect the measuring conditions, as would be the case with a rotating magnetic yoke by induced eddy currents.